In the process for electrolytic production of aluminium, such as by the Hall-Heroult process where aluminium is produced by reducing aluminium oxide in an electrolysis cell filled with melted electrolyte in the form of a fluoride-containing mineral to which aluminium oxide is supplied, the process gases comprises fluoride-containing substances such as hydrogen fluoride and fluorine containing dust. As these substances are extremely damaging to the environment, they have to be separated before the process gases can be discharged into the surrounding atmosphere. At the same time the fluorine-containing melt is essential to the electrolytic process, and it is desirable to recover the compounds for recirculation to the electrolysis. This recirculation may take place by adsorption of the fluorine-containing substances on a particulate adsorbent.
The system for recovery of the fluoride compounds comprises a filter system, which is included in a closed system. It is important to have stable transport of the gases from the aluminium production to the filter system. This transport is accomplished in gas ducts where the gases, by means of large fans, are conveyed through the gas ducts, comprising main ducts and branch ducts, to the filter system. For each aluminium production cell a branch duct is brought into the main duct, the cross section of the main duct increases gradually, by means of diffusers as the gas quantity increases. It is very important for the process as well as the environment that the gas distribution is as even as possible, and traditionally this is achieved by an increasingly stronger throttling of the gas in the branch duct the closer to the suction fans the branch duct is localised. Throttling represents sheer energy loss through a pressure drop. By the present invention, this pressure drop is substantially reduced, contributing to a reduced total pressure drop in the system. The total pressure drop in the duct system is measured from the first suction point. The invention may equally well be applied for gas ducts where there is a need for a different, but controlled, gas quantity from each suction point.
Previously it is known within the aluminium industry to bring the branch ducts with an angle of 30–90° into the main duct. The angular deviation causes slip and turbulence in the zone after the introduction of the gas. Previously it is also known to convey the gas through the branch duct with a velocity lower than the velocity in the main duct. This implies that the gas in the main duct must accelerate the gas from the branch duct. Thus the angular deviation, and the difference in the velocity causes an increased resistance in the main duct.
The duct system contributes to approximately 50% of the total pressure drop in the system for recovery of fluorides, this implies that a reduction in the pressure drop here will result in a considerably reduced operational cost for the plant and this gives the basis for the present invention. The aluminium industry is applied as an example, however, this is also a preferred field.
From SE 466 837 it is known branch ducts where the gas is guided into the main duct in parallel with the gas flow in the main duct. However, in said patent it is important that the velocity of the gas in the main duct and in the branch duct are principally the same, so that there is a low resistance both in the main duct and the branch duct.
It has now been found that a considerable reduction of the pressure drop in the gas duct, and consequently the energy consumption for the gas transport, may be achieved by carrying out the introduction of the gas from the branch duct in a new manner. The gas is guided, as in SE 466 837 into the main duct with a flow direction parallel to the flow of gas in the main duct. Through the first part of the branch duct, the velocity of the gas is lower than in the main duct. When the direction of the gas flow has been adjusted, being parallel with the direction of the gas flow in the main duct, the cross section is narrowed before the outlet of the branch duct by means of an nozzle, so that the gas is accelerated and the gas introduced into the main duct at a velocity higher than in the main duct. By this procedure, the pressure drop in the main duct, and the total energy requirement for the gas transport is considerably reduced. An even suction from each electrolysis cell is assured by adjusting the nozzle of the branch duct, which might be equipped with an adjustable flap. The examples being described relates to transport of process gases within the aluminum industry, but it is obvious for the person skilled in the art, that the same system for transport of gas may be utilised within all fields where there is a need for transport of gas from several points, e.g. other metallurgical industry, suction in laboratories, ventilation systems, etc. Further it is obvious for the person skilled in the art that the invention may be utilised also where there is need for gas transport with different but controlled gas quantities from each point of suction along a long duct.